Hi Karsten,
I use your first scenario now but the AnyConnect still says the port 443 is in use.
The port forwarding is set exclusive on Internet 2 (WAN 2) with a different fixed WAN IP and adapted DNS records. This forwarding is working fine on the WAN 2.
The AnyConnect setup wizard should detect that the WAN1 does NOt have any port forwarding going on on 443, but it does not...